1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device and a method of forming the same, and more particularly, it relates to a nitride-based semiconductor light-emitting device including an electrode layer and a method of forming the same.
2. Description of the Background Art
A nitride-based semiconductor laser element, which is an exemplary nitride-based semiconductor light-emitting device, is expected as the light source for an advanced large capacity optical disk and actively developed nowadays. In order to reduce the operating voltage of the nitride-based semiconductor laser element and improve reliability thereof, the contact resistance of electrodes must essentially be reduced. In particular, a nitride-based semiconductor has a low p-type carrier concentration and hence it is difficult to obtain an excellent ohmic property (low contact resistance) in relation to a p-side electrode. In order to cope with this, a Pd-based electrode material such as a Pd/Au electrode or a Pd/Pt/Au electrode containing Pd having an excellent ohmic property has been recently employed as the p-side electrode.
FIG. 28 is a sectional view showing a conventional nitride-based semiconductor laser element 150 having a Pd-based electrode. The structure of the conventional nitride-based semiconductor laser element 150 is now described with reference to FIG. 28. In the conventional nitride-based semiconductor laser element 150, an AlGaN low-temperature buffer layer 102 having a thickness of about 15 nm is formed on a sapphire substrate 101. An undoped GaN layer 103 having a thickness of about 3 xcexcm is formed on the AlGaN low-temperature buffer layer 102. An n-type GaN contact layer 104 is formed on the undoped GaN layer 103 with a thickness of about 5 xcexcm. An n-type AlGaN cladding layer 105 having a thickness of about 1 xcexcm, an MQW (multiple quantum well) emission layer 106 of InGaN having a thickness of about 50 nm and a p-type AlGaN cladding layer 107 of about 300 nm in thickness having a projection portion are formed on the n-type GaN contact layer 104. A p-type GaN contact layer 108 having a thickness of about 70 nm is formed on the projection portion of the p-type AlGaN cladding layer 107.
A p-side electrode 109 consisting of a Pd-based electrode having a three-layer structure of a Pd layer of about 10 nm in thickness, an Au layer of about 100 nm in thickness and an Ni layer of about, 200 nm in thickness in ascending order is formed on the p-type GaN contact layer 108. An SiO2 film 110 is formed to cover regions other than part of the upper surface of the p-side electrode 109 and the upper surface of the n-type GaN contact layer 104. A pad electrode 111 is formed to cover the SiO2 film 110 while coming into contact with the upper surface of the p-side electrode 109.
Partial regions of the layers from the p-type AlGaN cladding layer 107 to the n-type GaN contact layer 104 are removed. An n-side electrode 112 is formed to be in contact with an exposed part of the upper surface of the n-type GaN contact layer 104. A pad electrode 113 is formed to be in contact with the n-side electrode 112.
FIGS. 29 to 33 are sectional views for illustrating a process of fabricating the conventional nitride-based semiconductor laser element 150 having the Pd-based electrode shown in FIG. 28. FIG. 34 is a sectional view showing the conventional nitride-based semiconductor laser element 150 shown in FIG. 28 in a state mounted on a submount 170 from the side of an active layer in a junction-up system. The fabrication process for the conventional nitride-based semiconductor laser element 150 having the Pd-based electrode is now described with reference to FIGS. 28 to 34.
First, the AlGaN low-temperature buffer layer 102 is grown on the sapphire substrate 101 by MOCVD under a low temperature condition of about 600xc2x0 C. in order to relax lattice mismatching, as shown in FIG. 29. The undoped GaN layer 103 is formed on the AlGaN low-temperature buffer layer 102 with the thickness of about 3 xcexcm by MOCVD.
Thereafter the n-type GaN contact layer 104 having the thickness of about 5 xcexcm, the n-type AlGaN cladding layer 105 having the thickness of about 1 xcexcm, the MQW emission layer 106 having the thickness of about 50 nm, the p-type AlGaN cladding layer 107 having the thickness of about 300 nm and the p-type GaN contact layer 108 having the thickness of about 70 nm are successively formed on the undoped GaN layer 103 by MOCVD.
Then, partial regions of the layers from the p-type GaN contact layer 108 to the n-type GaN contact layer 104 are removed by anisotropic dry etching, as shown in FIG. 30.
Then, the Pd layer of about 10 nm in thickness, the Au layer of about 100 nm in thickness and the Ni layer of about 200 nm in thickness are formed in ascending order in a striped shape of about 2 xcexcm in width by a lift off method or the like, thereby forming the p-side electrode 109 consisting of the Pd-based electrode having the three layer structure of the Pd layer, the Au layer and the Ni layer as shown in FIG. 31. Thereafter the uppermost Ni layer of the p-side electrode 109 is employed as an etching mask for etching the p-type GaN contact layer 108 by anisotropic dry etching employing CF4 gas while etching the p-type AlGaN cladding layer 107 by about 150 nm. Thus, a ridge portion is formed as shown in FIG. 32.
Then, the SiO2 film 110 is formed on the overall surface by plasma CVD, and thereafter partially removed from the part of the n-type GaN contact layer 104 as shown in FIG. 33. The n-side electrode 112 is formed on the part of the n-type GaN contact layer 104 from which the SiO2 film 110 is partially removed.
Finally, the SiO2 film 110 is partially removed from the upper surface of the p-side electrode 109 consisting of the Pd-based electrode, followed by formation of the pad electrodes 111 and 113 on the p-side electrode 109 and the n-side electrode 112, as shown in FIG. 28.
The nitride-based semiconductor laser element 150 shown in FIG. 28 is fixed onto the submount (heat radiation base) 170 fixed to a stem 171 with a fusing material 160 such as solder, as shown in FIG. 34. In this case, the surface (the back surface of the sapphire substrate 101) of the element 150 opposite to the ridge portion is welded to the submount 170 in the junction-up system.
The conventional nitride-based semiconductor laser element 150 having the p-side electrode 109 consisting of the Pd-based electrode is formed in the aforementioned manner.
In the aforementioned conventional nitride-based semiconductor laser element 150 having the p-side electrode 109 consisting of the Pd-based electrode, however, the p-side electrode 109 consisting of the Pd-based electrode tends to peel during the fabrication process due to weak adhesion to the p-type GaN contact layer 108. Therefore, it is disadvantageously difficult to improve reliability of the element 150.
In the conventional nitride-based semiconductor laser element 150 having the p-side electrode 109 consisting of the Pd-based electrode, further, the contact property of the p-side electrode 109 is disadvantageously deteriorated due to heat or stress in a step of forming the pad electrode 112 on the p-side electrode 109 or an assembling step. In this case, contact resistance is so increased as to disadvantageously increase the operating voltage.
An object of the present invention is to provide a highly reliable nitride-based semiconductor light-emitting device having low operating voltage.
Another object of the present invention is to increase adhesion of the whole of an electrode layer to a nitride-based semiconductor layer without damaging a low contact property in the aforementioned nitride-based semiconductor light-emitting device.
Still another object of the present invention is to provide a method of forming a nitride-based semiconductor light-emitting device capable of easily forming a highly reliable nitride-based semiconductor light-emitting device having low operating voltage.
A nitride-based semiconductor light-emitting device according to a first aspect of the present invention comprises a nitride-based semiconductor layer formed on an active layer and an electrode layer partially formed on the nitride-based semiconductor layer, while the electrode layer includes a first electrode layer containing a material having strong adhesion to the nitride-based semiconductor layer and a second electrode layer formed on the first electrode layer to have a portion coming into contact with the surface of the nitride-based semiconductor layer with weaker adhesion to the nitride-based semiconductor layer than the first electrode layer for reducing contact resistance of the electrode layer to the nitride-based semiconductor layer.
As hereinabove described, the nitride-based semiconductor light-emitting device according to the first aspect is partially provided on the nitride-based semiconductor layer with the first electrode layer containing the material having strong adhesion to the nitride-based semiconductor layer and provided on the first electrode layer with the second electrode layer, reducing contact resistance to the nitride-based semiconductor laser, having the portion coming into contact with the surface of the nitride-based semiconductor layer, whereby the first electrode layer can increase adhesion of the overall electrode layer to the nitride-based semiconductor layer while the second electrode layer can attain low contact resistance. Thus, reliability of the device can be improved while operating voltage can be reduced.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the second electrode layer preferably has lower contact resistance to the nitride-based semiconductor layer than the first electrode layer. According to this structure, the second electrode layer can easily attain low contact resistance.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the first electrode layer preferably contains at least one material selected from a group consisting of Pt, Ni, Cr, Ti, Hf and Zr, and the second electrode layer preferably contains Pd. In this case, the first electrode layer more preferably includes a Pt layer, and the second electrode layer more preferably includes a multilayer film having a Pd layer. According to this structure, the first electrode layer can easily improve adhesion of the overall electrode layer to the nitride-based semiconductor layer, and the second electrode layer can easily attain low contact resistance.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the first electrode layer is preferably partially formed on the nitride-based semiconductor layer in heterogeneous distribution. According to this structure, the second electrode layer can come into contact with the nitride-based semiconductor layer on a region of the nitride-based semiconductor layer formed with no first electrode layer, whereby contact resistance of the second electrode layer can be easily reduced.
In this case, the first electrode layer is preferably formed on the nitride-based semiconductor layer with a thickness of not more than 3 nm. According to this structure, the first electrode layer can be easily formed on the nitride-based semiconductor layer with islandlike heterogeneous distribution.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the first electrode layer is preferably formed by patterning. According to this structure, the first electrode layer can be partially formed on a prescribed region of the nitride-based semiconductor layer.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the nitride-based semiconductor layer preferably has an irregular surface. According to this structure, the contact areas between the nitride-based semiconductor layer and the first and second electrode layers can be increased, whereby the contact area between the nitride-based semiconductor layer and the second electrode layer can be inhibited from reduction resulting from formation of the second electrode layer on the nitride-based semiconductor layer through the first electrode layer. Thus, contact resistance can be stably reduced. In this case, the nitride-based semiconductor layer having the irregular surface preferably has an In composition of at least 3% and a thickness of not more than 20 nm. When the nitride-based semiconductor layer is formed with such a composition and such a thickness, the surface of the nitride-based semiconductor layer can be easily irregularized.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the nitride-based semiconductor layer preferably includes a contact layer formed on a projection portion of a cladding layer, and the projection portion of the cladding layer and the contact layer preferably form a ridge portion. While an electrode must be formed on the contact layer having a narrow area in this structure, the first electrode layer can improve adhesion of the overall electrode layer to the contact layer forming the ridge portion and the second electrode layer can attain low contact resistance. Consequently, operating current and operating voltage can be reduced, whereby the device can be improved in reliability.
In the aforementioned nitride-based semiconductor light-emitting device according to the first aspect, the second electrode layer preferably has a lower energy barrier to the nitride-based semiconductor layer than the first electrode layer. According to this structure, the second electrode layer can easily attain low contact resistance.
A nitride-based semiconductor light-emitting device according to a second aspect of the present invention comprises a nitride-based semiconductor layer formed on an active layer and an electrode layer formed on the nitride-based semiconductor layer, while the electrode layer includes a first electrode layer containing a material having strong adhesion to the nitride-based semiconductor layer and a second electrode layer formed on the first electrode layer to have a portion coming into contact with the surface of the nitride-based semiconductor layer with weaker adhesion to the nitride-based semiconductor layer than the first electrode layer for reducing an energy barrier of the electrode layer to the nitride-based semiconductor layer.
The nitride-based semiconductor light-emitting device according to the second aspect is partially provided on the surface of the nitride-based semiconductor layer with the first electrode layer containing the material having strong adhesion to the nitride-based semiconductor layer and provided on the first electrode layer with the second electrode layer reducing the energy barrier to the nitride-based semiconductor layer as hereinabove described, whereby the first electrode layer can improve adhesion of the overall electrode layer to the nitride-based semiconductor layer and the second electrode layer can attain low contact resistance. Thus, the device can be improved in reliability and operating voltage can be reduced.
In the aforementioned nitride-based semiconductor light-emitting device according to the second aspect, the second electrode layer preferably has lower contact resistance to the nitride-based semiconductor layer than the first electrode layer. According to this structure, the second electrode layer can easily attain low contact resistance.
In the aforementioned nitride-based semiconductor light-emitting device according to the second aspect, the first electrode layer preferably contains at least one material selected from a group consisting of Pt, Ni, Cr, Ti, Hf and Zr, and the second electrode layer preferably contains Pd. In this case, the first electrode layer more preferably includes a Pt layer, and the second electrode layer more preferably includes a multilayer film having a Pd layer. According to this structure, the first electrode layer can easily increase adhesion of the overall electrode layer to the nitride-based semiconductor layer, and the second electrode layer can attain low contact resistance.
In the aforementioned nitride-based semiconductor light-emitting device according to the second aspect, the first electrode layer is preferably partially formed on the nitride-based semiconductor layer in heterogeneous distribution. According to this structure, the second electrode layer can come into contact with the nitride-based semiconductor layer on a region of the nitride-based semiconductor layer formed with no first electrode layer, whereby the second electrode layer can easily reduce contact resistance.
In this case, the first electrode layer is preferably formed on the nitride-based semiconductor layer with a thickness of not more than 3 nm. According to this structure, the first electrode layer can be easily formed on the nitride-based semiconductor layer with islandlike heterogeneous distribution.
In the aforementioned nitride-based semiconductor light-emitting device according to the second aspect, the first electrode layer is preferably formed by patterning. According to this structure, the first electrode layer can be partially formed on a prescribed region of the nitride-based semiconductor layer.
In the aforementioned nitride-based semiconductor light-emitting device according to the second aspect, the nitride-based semiconductor layer preferably has an irregular surface. According to this structure, the contact areas between the nitride-based semiconductor layer and the first and second electrode layers can be so increased that the contact area between the nitride-based semiconductor layer and the second electrode layer can be inhibited from reduction resulting from formation of the second electrode layer on the nitride-based semiconductor layer through the first electrode layer. Thus, contact resistance can be stably reduced. In this case, the nitride-based semiconductor layer having the irregular surface preferably has an In composition of at least 3% and a thickness of not more than 20 nm. When the nitride-based semiconductor layer is formed with such a composition and such a thickness, the surface of the nitride-based semiconductor layer can be easily irregularized.
In the aforementioned nitride-based semiconductor light-emitting device according to the second aspect, the nitride-based semiconductor layer preferably includes a contact layer formed on a projection portion of a cladding layer, and the projection portion of the cladding layer and the contact layer preferably form a ridge portion. While an electrode must be formed on the contact layer having a narrow area in this structure, the first electrode layer can increase adhesion of the overall electrode layer to the contact layer forming the ridge portion and the second electrode layer can attain low contact resistance. Consequently, operating current and operating voltage can be reduced, whereby the device can be improved in reliability.
A method of forming a nitride-based semiconductor light-emitting device according to a third aspect of the present invention comprises steps of forming a nitride-based semiconductor layer on an active layer, partially forming a first electrode layer containing a material having strong adhesion to the nitride-based semiconductor layer on the surface of the nitride-based semiconductor layer, and forming a second electrode having weaker adhesion to the nitride-based semiconductor layer than the first electrode layer and containing a material having lower contact resistance to the nitride-based semiconductor layer than the first electrode layer on the first electrode layer to have a portion coming into contact with the surface of the nitride-based semiconductor layer.
In the method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the first electrode layer containing the material having strong adhesion to the nitride-based semiconductor layer is partially formed on the surface of the nitride-based semiconductor layer and the second electrode layer containing the material having low contact resistance to the nitride-based semiconductor layer is formed on the first electrode layer to have the portion coming into contact with the surface of the nitride-based semiconductor layer as hereinabove described, whereby the first electrode layer can increase adhesion of the overall electrode layer to the nitride-based semiconductor layer and the second electrode layer can attain low contact resistance. Thus, reliability of the device can be improved and the nitride-based semiconductor light-emitting device can be easily formed to be capable of reducing operating voltage.
In the method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of partially forming the first electrode layer on the surface of the nitride-based semiconductor layer preferably includes a step of forming the first electrode layer on the surface of the nitride-based semiconductor layer with a small thickness partially forming an opening. According to this structure, the first electrode layer can be easily partially formed on the surface of the nitride-based semiconductor layer.
In the method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of partially forming the first electrode layer on the surface of the nitride-based semiconductor layer preferably includes a step of forming the first electrode layer and the second electrode layer on the surface of the nitride-based semiconductor layer and thereafter feeding current between the second electrode layer and the nitride-based semiconductor layer thereby moving part of the second electrode layer to come into contact with the surface of the nitride-based semiconductor layer. According to this structure, the second electrode layer can reliably exhibit a low contact property without damaging adhesion of the first electrode layer to the nitride-based semiconductor layer.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of partially forming the first electrode layer on the surface of the nitride-based semiconductor layer preferably includes steps of forming a resist pattern on the surface of the nitride-based semiconductor layer, and forming the first electrode layer on the resist pattern and thereafter removing the resist pattern thereby patterning the first electrode layer. When a lift off method is employed in this manner, the first electrode layer can be easily partially formed on the surface of the nitride-based semiconductor layer. Thus, the second electrode layer can reliably come into contact with the nitride-based semiconductor layer on a region of the nitride-based semiconductor layer formed with no first electrode layer.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of partially forming the first electrode layer on the surface of the nitride-based semiconductor layer preferably includes steps of forming the first electrode layer on the surface of the nitride-based semiconductor layer and thereafter forming a resist pattern on the first electrode layer, and etching the first electrode layer through the resist pattern serving as a mask thereby patterning the first electrode layer. According to this structure, the first electrode layer can be more reliably partially formed on the surface of the nitride-based semiconductor layer. Thus, the second electrode layer can reliably come into contact with the nitride-based semiconductor layer on a region of the nitride-based semiconductor layer formed with no first electrode layer.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device patterning the first electrode layer, the step of forming the first electrode layer preferably includes a step of forming the first electrode layer in a lattice shape in plan view by patterning the first electrode layer. According to this structure, the first electrode layer can be partially formed on the surface of the nitride-based semiconductor layer with homogeneous distribution, whereby the first electrode layer can homogeneously increase adhesion of the overall electrode layer to the nitride-based semiconductor layer in the in-plane direction of the nitride-based semiconductor layer. When the first electrode layer is formed in the lattice shape, the second electrode layer can reliably come into contact with the nitride-based semiconductor layer on the region of the nitride-based semiconductor layer formed with no first electrode layer.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of forming the first electrode layer preferably includes a step of forming the first electrode layer by any of electron beam heating evaporation, resistance heating evaporation and sputter deposition. When employing such a method, the first electrode layer containing the material having strong adhesion to the nitride-based semiconductor layer can be easily formed. Further, the first electrode layer can be easily formed on the nitride-based semiconductor layer with islandlike heterogeneous distribution.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of forming the second electrode layer preferably includes a step of forming the second electrode layer on the first electrode layer and thereafter performing heat treatment. According to this structure, contact resistance of the second electrode layer can be further reduced.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the nitride-based semiconductor layer preferably includes a contact layer formed on a cladding layer, and the step of forming the second electrode layer preferably includes a step of forming the second electrode layer on a prescribed region of the upper surface of the first electrode layer by a lift off method, while the method preferably further comprises a step of forming the second electrode layer and thereafter partially etching the first electrode layer, the contact layer and the cladding layer through the second electrode layer serving as a mask thereby forming a ridge portion. According to this structure, the ridge portion consisting of a projecting portion of the cladding layer and the contact layer can be easily formed. When the first electrode layer is patterned not by the lift off method but by etching, pattern peeling easily caused in the lift off method can be prevented.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the first electrode layer preferably contains at least one material selected from a group consisting of Pt, Ni, Cr, Ti, Hf and Zr, and the second electrode layer preferably contains Pd. According to this structure, the first electrode layer can easily increase adhesion of the overall electrode layer to the nitride-based semiconductor layer, and the second electrode layer can attain low contact resistance.
In the aforementioned method of forming a nitride-based semiconductor light-emitting device according to the third aspect, the step of forming the nitride-based semiconductor layer preferably includes a step of forming the nitride-based semiconductor layer having an irregular surface. According to this structure, the contact areas between the nitride-based semiconductor layer and the first and second electrode layers can be so increased that the contact area between the nitride-based semiconductor layer and the second electrode layer can be inhibited from reduction resulting from formation of the second electrode layer on the nitride-based semiconductor layer through the first electrode layer. Thus, contact resistance can be stably reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.